Abstract: The flow characteristics of carbon fiber fabric was studied under different forming conditions, including fabric type, fiber volume fraction and ultrasonic vibration. The unidirectional permeability measurement device of ultrasonic vibration assisted RTM was newly designed. Permeability testing experiment including 16 trials was carried out, and the resin flow characteristics in the fabric were analyzed using COMSOL simulation. Results show that with the same fiber volume fraction, the fiber bundle gap channel of 2/2 twill weave fabric is wider than that of plain weave fabric, and the permeability of twill weave fabric is averagely 21.5% higher than result from plain weave fabric. The fiber volume fraction is negatively correlated with fabric permeability, and the functional relationship is in good agreement with the semi-empirical Kozeny-Carman (K-C) equation. The ultrasonic vibration is introduced into the resin flow process, and the ultrasonic cavitation effect, acceleration effect and micro-jet effect acted on the surface of fiber bundle, which significantly improves the permeability of fiber fabric about 58.2%. The flow process of fabric model, which is designed with elliptic and nearly rectangular fiber bundle section, was simulated by finite element model, and results show that the high velocity area of near rectangular fiber bundle section is relatively wider, and the velocity component of fluid impregnation to the interlayer of fiber cloth is larger. Ultrasonic acting on the fabric fibers probably drives the fiber bundle peristalsis, making the fiber bundle cross section tend to be nearly rectangular, subsequently improving the resin infiltration of the fiber fabric. Experimental results from this work have certain guiding significance to optimize the forming process of composites.